Silicone rubber compositions are used in a wide variety of fields by virtue of their advantages including ease of working, ease of molding, and excellent molded properties such as heat resistance, freeze resistance, weather resistance and electrical insulation. In particular, compositions based on fluorosilicone rubber having 3,3,3-trifluoropropyl groups as side chain have solvent resistance as well and find use as diaphragms, O-rings, oil seals and other parts in transportation equipment and petroleum-related equipment.
As opposed to dimethylsilicone, fluorosilicone is known susceptible to oxidative degradation and siloxane decomposition under the action of heat, acid and alkali. Presumably, such phenomena are caused by ease of polarization attributable to trifluoropropyl groups and hydrogen fluoride generated upon oxidation. As a result, when the fluorosilicone is used in contact with a non-silicone resin, its physical properties can be substantially impaired by migrants from the adjacent resin. For example, when the fluorosilicone is used in contact with nylon 6, it will be swollen in the reactant (i.e., raw material), ε-caprolactam. After swell, compression set is substantially exacerbated as compared with the unaffected state. This is because ε-caprolactam which is a polar substance serves to enhance the activity of trace alkali components (originating from additives and neutralization salt of polymerization catalyst), inducing scission of fluorosilicone rubber main chain.
Under the circumstances, fluorosilicone rubber compositions which are improved in compression set are proposed as disclosed in Patent Documents 1 to 3. Despite these proposals, the compression set after swell in ε-caprolactam is still unsatisfactory.